One procedure for fatigue testing of dental implants includes having a loading shaft 10, shown in FIG. 1, repeatedly apply a compressive load to an implant 12. The direction of the force vector that applies the compressive load defines a load axis 14, as shown in FIG. 1.
During testing, the implant 12 is implanted into a mounting surface 16. A line that extends from the mounting surface to a point at which the force is applied defines a moment arm 18. To the extent the loading shaft 10, and hence the force vector, is not parallel to this moment arm 18, there will be a torque that urges the implant 12 to rotate or bend.
The extent to which the implant 12 resists such rotation or bending is of considerable interest. In fact, ISO 14801 describes a testing procedure and a fixture to be used for such a procedure. According to section 5.2.1 of that standard, the loading force is to be applied such that no lateral constraint occurs. According to section 5.2.6, the loading device should be unconstrained in the transverse direction so as to avoid reducing the magnitude of the applied torque. The standard further specifies that this should be accomplished by either providing a universal joint or by point contact between whatever applies the loading force and the implant 12.
The application of point contact is difficult, and when an implant 12 fails the test, the loading member falls away unless otherwise suspended.
Testing machines that rely on a universal joint to comply with the ISO standard are known. However, inherent in a conventional universal joint is a backlash that results from a delay between the time a drive motor applies a force and the time at which this force is actually transmitted down a shaft. This delay arises from slack in the bearings used in the pivots of a universal joint. In part as a result of this, universal joints require considerable maintenance. However, even with diligent maintenance, under the grueling test conditions of repeatedly applying a compressive load to an implant 12, such joints have a tendency to fail prematurely. In addition, the wear on these bearings results in a constant dispersal of small metal particles or shavings during use.